I've written about this before.
Couple of weeks ago, I was shooting the US Track and Field Olympic Team Trials in Eugene, OR when I heard some guy start screaming profanity at the top of his lungs. He just had a $5K lens fall off a 5D Mkiii. I shared his pain ...
Commisserating later with him, over glasses of bourbon, he said that he rotated the camera from landscape to portrait and shot a couple of frames. He switched to another lens/body and when he went back to the 5D the lens came off in his hand and he dropped it.
We both use a classic grip with the left palm up supporting the body and lens, with the left thumb and fingers working the zoom ring. When the body is rotated 90 degrees clockwise from landscape to portrait, the over-size, lens release button with a weak spring passes right under the meaty part of your thumb.
The lens release button is raised above the body so that it is on the same plane as the lens mount. It then sticks up about another 1/4". The spring offers almost no resistance when the button is pushed. There's no detent position or click sound to indicate that it has been unlocked. The locking pin is about 1/16" in diameter and seats in a reciever hole in the rear of the lens. The pin only travels about 1/16" inch. So if the lens moves even a fraction of a degree when the button is pushed, the pin won't reseat in its hole and the lens is unlocked. Note that if the lens is only rotated a short distance, the data and electrical contacts are still touching each other and there will be no indication or error message in the viewfinder.
I don't use 5D's anymore because of this problem. I use these tools to feed my family. There's no excuse for accidental lens release on expensive bodies. This is a design flaw that needs to be corrected.
"As Robert said, once I know it is possible, I immediately stop using that kind of grip. Now I just put my palm underneath the lens and not curve around the lens and I'm conscious of the release button at all time. It was hard at first but now it's second nature."
Here's a typcial illustration found in instruction manuals. I don't think Canon is about to change the release button.
I found the only hard thing about holding the camera with the palm facing upwards was the weight of some lenses. I've always held a camera like what is shown in Canon's illustration.
The only time my pam faces downward is when the camera is mounted on a tripod, and my hand cannot reach underneath the lens.
[EDIT] Okay, I just picked up a 6D w/grip and a 7D2 w/o grip. With both cameras, the exact positioning of my left hand underneath the lens would vary with the lens, the positions of the zoom and focusing rings, and the size of the lens.
For big lenses that use a tripod foot, I use the foot as a "handle" and put it in the center of my palm, which balances the setup in my hands fairly well. For smaller lenses without a tripod foot. I've just noticed that the bottom left corner of the camera sits on the heel of my palm. This works for me because I have big hands.
